The modern data network space includes a plurality of federated domains, or areas, that are interconnected by a backbone network. FIG. 1 is a block diagram showing a typical network 2 having two areas 4 (Area 0.0.0.1 and Area 0.0.0.2) that are linked to a backbone network 6 via one or more respective Area Border Routers (ABRs) 8. Each area 4 includes one or more internal routers (IRs) 10, which control the forwarding of traffic among user machines 12 (e.g. PCs) and respective ABRs 8 hosting the area 4. Each of the routers 8,10 are coupled together via links 14 (which may be physical or logical links) through which packetized data is forwarded. Thus within area 0.0.0.1, traffic flows between ABR(A) 8a, and user machines 12 within area 0.0.0.1 are routed through IR(A) 10a via an intervening link 14a. Similarly, within area 0.0.0.2, traffic flows between ABR(B) 8b, and user machines 12 within area 0.0.0.2 are routed through IR(B) 10b via an intervening link 14b. 
The topology of the network 2 illustrated in FIG. 1 is typical of that set up within an enterprise or campus Local Area Network (LAN) to connect various domains (e.g. departmental LANs) represented by each area 4 to one another (and possibly to an external network such as the public internet, not shown). Typically, traffic forwarding within such a network 2 is controlled using an interior gateway protocol (IGP) such as Open Shortest Path First (OSPF).
The topology of FIG. 1 is vulnerable to resource failures affecting traffic flows between an area 4 and the backbone 6. In particular, in an event of failure of either ABR(A) 8 or the link 14a between IR(A) 10a and ABR(A) 8a, traffic flow between user machines 12 in area 0.0.0.1 and the backbone 6 (and thus also area 0.0.0.2) will be interrupted. Similarly, in an event of failure of either ABR(B) 8b, or the link 14b between IR(B) 10b and ABR(B) 8b, traffic flow between user machines 12 hosted by area 0.0.0.2 and the backbone 6 will be interrupted. One way of overcoming this vulnerability is to install back-up links 16 (shown in dashed lines) between IR(A) 10a and ABR(B) 8b, and between IR(B) 10b and ABR(A) 8a. In principle, the installation of such back-up links 16 in this manner can provide an alternate path for traffic flows between areas 0.0.0.1 and 0.0.0.2 and the backbone 6 in an event of failure of either one of ABR(A) 8a or ABR(B) 8b. 
However, under the OSPF protocol, the provisioning of a new link triggers conventional OSPF flooding of link State Advertisement (LSA) messages advertising the new link to adjacent routers. Once advertised, the new link becomes available for carrying traffic. This advertisement behavior inevitably results in each of the back-up links 16 being advertised to the network 2, and subsequently carrying data traffic, even when the ABRs 8 are operating normally. The amount of traffic flowing in these back-up links 16 may be reduced somewhat by manipulating the metrics assigned to these links 16. However, this typically requires manual intervention, which increases network management effort. Furthermore, the installation of back-up links 16 has the effect of making ABR(B) 8b part of area 0.0.0.1, and ABR(A) 8a part of area 0.0.0.2. Consequently, conventional OSPF will result in these routes being favored for inter-area traffic (e.g. traffic originating in area 0.0.0.1 and destined for area 0.0.0.2), regardless of the metrics assigned to each of the back-up links 16. In addition, because IR(A) 10a can forward traffic destined for area 0.0.0.2 directly through ABR(B) 8b (thereby bypassing ABR(A) 8a) the implementation of policies respecting the forwarding of traffic between areas 0.0.0.1 and 0.0.0.2 is made significantly more complicated. Thus, the insertion of back-up links 16 in this manner tends to increase the complexity of the network topology, and complicates network management and administration.
Accordingly, a method and apparatus for providing protection against resource failures in Open Shortest Path First (OSPF) networks, without increasing the complexity of the network topology and/or management, remains highly desirable.